Granular polytetrafluoroethylene of good moldability and apparent bulk density

ABSTRACT

Granular polytetrafluoroethylene is ground to a particle size of about 20-50 microns, and slurried in water at a temperature less than 40* C. to increase the apparent bulk density of the polymer without a substantial change to the moldability index and particle size thereof.

United States Patent Leverett Us] 3,690,569 [451 Sept. 12, 1972 [54]GRANULAR POLYTETRAFLUOROETHYLENE OF GOOD MOLDABILITY AND APPARENT BULKDENSITY [72] Inventor: Glenn Frederick Leverett, Vienna,

W. Va.

[73] Assignee: E. I. du Pont de Nemours and Company, 2, Wilmington, Del.

[22] Filed: Sept. 24, 1970 [21] Appl. No.: 75,235

[52] U.S. Cl. ..24l/17, 241/21, 241/29,

260l92.1 [51] Int. Cl. ..B02c 19/00, B02c 23/00, C08f 3/24 [58] Field ofSearch ..24lll7, 21; 29/30; 260/921 [56] References Cited UNITED STATESPATENTS 3,115,486 12/1963 Weisenberger ..260/92.1

3,535,301 10/1910 G'roppelli ..'..260/92.l

3,513,144 5/1970 Kometani ..'.260/92.1 3,150,834 7/1964 Doyle; ..241/21XPrimary Examiner-Wayne A. Morse, Jr. Attorney-Edwin Tocker [57] ABSTRACTGranular polytetrafluoroethylene is ground to a parti' 9 Claims, N0Drawings This invention relates to granular polytetrafluoroethylene andmore particularly to a process for increasing the apparent bulk densitythereof.

The polymerizationof tetrafluoroethylene to the granular type ofpolytetrafluoroethylene (as distinguished from the aqueous dispersion orfine powder type) is disclosed in U.S. Pat. No. 2,393,967 to Brubaker.Briefly, the polymerization is conducted by pressuring gaseoustetrafluoroethylene into an agitated aqueous medium containing dissolvedpolymerization initiator and a buffer. The agitation of the mediumcauses the polymer as it is formed to settle to the bottom of thereaction vessel, and the resultant polymer is obtained in the form ofrelatively large particles.

Early in the development of granular polytetrafluoroethylene, it wasfound that for general purpose molding these large polymer particles hadto be comminuted. While comminution did improve the ability of thepolymer to be used as a molding powder, it introduced the problem ofpowder flow; i.e., the molding powder would not flow uniformly to fill amold. In addition, the quality of the molded article was lower thendesired for many applications.

The poor powder flow character of this early comminuted polymer wasgreatly improved upon by the invention of Weisenberger disclosed in U.S.Pat. No. 3,115,486, which involved comminuting the granular polymer toparticle sizes in the range of 200 to 700 microns (wet sieve) followedby slurrying the resultant powder in water at elevated temperatures of40 to 70C. The poor moldability character of the early comminuted powderwas greatly improved upon by comminuting to still smaller particlesizes, such as less than 50 microns (wet sieve), as disclosed in U. S.Pat. No. 2,936,301 to Thomas and Wallace. Unfortunately, while theWeisenberger invention improved powder flow, the quality of theresultant molded article was not as high as desired. Similarly, whilefiner grinding such as disclosed by Thomas and Wallace improved thequality of the molded article, the powder of the finer ground powder wasnot as good as desired.

It was next discovered that by slurrying the finer ground granularpolymer (as prepared by the Thomas and Wallace method) in water attemperatures above 40C., a powder with both good powder flow and bettermoldability was obtained. This discovery is described in British Pat.No. 1,033,638 to Du Pont (see also U. S. Pat. No. 3,366,615). However,the quality of the molded article, although improved, was still not asgood as that which was obtained by only fine grinding (such as by theinvention of Thomas and Wallace). This was due to the increase inparticle size that occurred during the slurrying in hot water.

One of the effects of the slurrying treatment of the powder in theaforementioned British patent was to increase the apparent bulk densityof the powder. This had the beneficial result of requiring less moldvolume for a given weight of powder. To illustrate the significance ofthis benefit, the earliest granular polytetrafluoroethylene moldingpowder required a 40 foot high mold cavity to produce a molded article 4foot high. In the treatment disclosed in the British patent, theincreased apparent bulk density of the powder was obtained at theexpense of giving up some quality in the molded article due to theincrease in particle size.

The present invention involvesthe discovery that by slurrying the finerground granular polytetrafluoroethylene in water at a temperature lessthan 40C., the resultant molding powder not only increases in apparentbulk density, but also retains its original moldability and particlesize.

The expression moldability as used herein to refer to the quality of thearticle molded from granular polytetrafluoroethylene can bequantitatively defined by such direct measurements as tensile strengthand elongation. More often, however, such indirect tests as voidcontent, dielectric strength, or specific gravity of the molded articlerelative to the standard specific gravity of the resin are used. Thesetests measure the degree of compactness of the article, with the degreeof compactness being directly related to the strength and toughness ofthe article. Moldability will be defined herein by the term moldabilityindex which is the difference between the standard specific gravity(886) of the polytetrafluoroethylene, as determined by the procedure ofASTM D-1457-56T, and the specific gravity of a sample of the same powderpreformed at 1,000 psig. instead of the 5,000 psig. pressure of the ASTMprocedure, the difference between the two specific gravities beingmultiplied by 1,000 to obtain the moldability index. The smaller themoldability index, the closer together are the specific gravitiesinvolved, which in turn indicates the lower porosity and thus, thehigher quality of the molded article.

The apparent bulk density of the molding powder is obtained by theprocedure of ASTM D-l457-5 6T. Units are in grams per liter.

The particle size disclosed herein (unlessotherwise indicated) of themolding powder is the average or d particle size diameter in microns asdetermined by the Micromerograph technique wherein the air sedimentationrate of a representative sample of the molding powder is determinedusing the procedure and apparatus disclosed in U.S. Pat. No. 2,597,099,and this is converted to particle size distribution using Stokes Lawwhich relates particle size to rate of fall. By-

plotting the cumulative weight percent of material versus particlediameter a bell curve is obtained with the apex of the curverepresenting the d particle diameter or average particle diameter. Theair sedimentation test is carried out in a column providing a free-falldistance of eight feet. The distribution of particle diameters is suchthat usually at least about percent by weight of the powder diametersfall within 3:20 microns of the d value. The d particle size determinedby this technique is about the same as would be obtained for the wetsieve size procedure disclosed in the Thomas and Wallace patent. v

The polytetrafluoroethylene starting material is the granular type ofpolymer. This type of polymer can be prepared by a procedure such asdescribed in the Brubaker patent, wherein the polymer has a specificsurface area of less than 3 square meters per gram. The starting polymercan also be made by the modification of the Brubaker process disclosedin U.S. Pat. No.

3,245,972 to Anderson, Edens, and Larson wherein nuclei are produced inthe early stage of the granular polymerization process and these nucleilead to the for- I mation of granular polytetrafluoroethylene having anincreased specific surface area of from 3-9 square metersper gram. Onemethod for nucleation is to aid to the polymerization medium from 50 to500 ppm. (based on the total weight of the aqueous medium) oftelogenically inactive fluorinated dispersing agent. at the beginning ofthe polymerization reaction. Another method for making thepolytetrafluoroethylene starting material, to obtain a stillgreaterincrease in specific surface, is minimize or eliminate thepresence of buffer, which is typically an ammonium compound, that ispresent in the polymerization medium, as disclosed in German Pat.Publication No. 1,950,767 (U.S. Ser. No. 766,314, filed Oct. 9, 1968 byEsker). Specific surface area of the polymer is measured by nitrogenabsorption, for example, by the procedure disclosed in Chapter XII of W.E. Barr and V. J. Anhorn, Scientific and Industrial Glass Blowing andLaboratory Techniques, instruments Publishing Company 1949).

parent bulk density of the polymer powder to be in excess of 400 gramsper liter, and preferably in excess of 450 grams per liter. Depending onthe polymer concentration in the water and degree of turbulence, theresidence time required to reach the bulk density desired will generallybe at least 0.5 minute, and usually no significant benefit is obtainedby slurrying for more than 10 hours. The slurrying can be conductedbatchwise or continuous by having the inlet and outlet to the agitationequipment used appropriately sized and located to prevent fresh feed tothe tank from short-circuiting the desired agitation time.

The particle size of the starting polymer will depend on the;polymerization process by which it is made; generally the particle sizewill be 500 microns (wet sieve) or greater. In any event, the polymer iscomminuted to a particle size of less than 100 microns and generally inthe range of 10 to, 75 microns but preferably to a'particle size of 20to 50 microns, using any comminuting equipment which is capable ofproducing the small particle sizes desired. For example, suitableequipment include micronizers, jet-o-mizer, and the apparatus disclosedin the Thomas and Wallace patent.

Apparatus such as the Fitzmill disclosed in the Weisenberger patent andWaring blenders which have a bladed stirrer operating at 13,000 rpm, asdisclosed in the Anderson, Edens, and Larson patent, are incapable ofproducing this result.

The comminution can be'carried out dry or with the polymer contained inan aqueous medium. Preferred comminution temperatures are in the rangeof 0 to 40 C., but preferably at a temperature of less than 15C.Generally the apparent bulk density of the comminuted polymer will befrom 200 to 425 grams per liter.

Following comminution, the finely ground powder is added to water whichis at a temperature of less than 40C., and the resultant combination isagitated. By agitation is meant that turbulence is produced in thecombination of polymer and water, whereby the polymer particles, whichare not water wet, disperse within the water, probably producingparticle-to-particle contact, to form a slurry. Equipment for producingthis result, typically baffled tanks equipped with a stirrer, are wellknown in the art.

If the comminution is carried out wet, i.e., on a slurry of the powderin water, the slurry can be merely passed from the cutter to a slurryingtank operating under the aforementioned condition of agitation. In anyevent, sufficient water is present during the agitation step to form theslurry. Typically, from 3 to 20 parts of water will be present for eachpart of polymer powder, all

parts being by weight.

The residence time of the polymer powder in the slurrying tank shouldbe'sufficient to increase the ap- The temperature of slurrying iscritical. Preferably, the slurrying temperature is no greater than 30C.The slurrying can also be carried out at a water temperature of lessthan 19C. which is the temperature of transition of the triclinic form(19C. and below) to the hexagonal form. At the relatively lowtemperatures of 30C. and less, the agitation apparently does notagglomerate the particles into larger particles which are stable insubsequent handling. This is contrary to the agglomeration resultsdisclosed in the aforementioned British patent upon slurrying attemperatures of 40 to C. The lowest slurrying temperature isabout 0C.where the formation of ice interferes with the agitation.

The slurrying process of the present invention can also be operated inthe range of 30C. to less than 40C. with marginal improvement, dependingon the time of slurrying. At short slurrying times, most, if not all,the original moldability index and particle size of the comminutedpolymer is retained; but the ability to retain moldability index andparticle size diminishes as the slurrying temperature approaches 40C. Asthe slurrying time decreases (to save the original moldability index),there is a smaller increase in apparent bulk density. By extending theslurrying time, some further increase in apparent bulk density with onlyminor loss of moldability (increase in moldability index) and increasein particle size can be obtained. The degree of loss that can betolerated will depend on the moldability index and particle size of thestarting comminuted polymer and properties desired in the final product.Typically, the agitation will be conducted under conditions at whichneither the moldability index nor particle size increase by more than 20percent based on their starting values. More often than not, especiallyat slurry temperatures of no greater than 30C., one or both of theseparameters will not increase and may decrease.

Following slurrying, the water and powder can be separated from oneanother and the powder can be dried at a temperature less than thesintering temperature (327C.) of the resin. The usual drying techniqueis to expose the powder to circulating air heated to a temperature up to300C.

Alternatively, the wet powder can be passed to the same or differentcomminuting equipment for one or more additional cycles of comminutionand slurrying. The comminution in each successive cycle has a smalleffect on reducing the particle size of the polymer relative to theeffect of the first comminution, but each comminution increases themoldability (decreased the moldability index) of the powder whilelowering its apparent density, and the slurrying step in each cycleincreases the apparent bulk density. Generally, a better combination ofbulk density and moldability index is obtained by subjecting the polymerpowder to two or more cycles of comminution and slurrying than toprolong the comminution an slurrying of a singlecycle for an equivalentamount of time.

The resultant polymer has good powder flow and is useful in moldingapplications in general in the same way as the granularpolytetrafluoroethylene made heretofore. The powder is especially usefulin making large molded articles because the increasedbulk density of thepowder requires a smaller mold volume.

Specific embodiments of the invention are as follows (parts and percentsare by weight unless otherwise indic ated):

EXAMPLE 1 The granular polytetrafluoroethylene used in this Example wasprepared following the procedure of Example of the aforementioned U.S.Pat. application Ser. No. 766,314 and had a specific surface area of 3.5square meters per gram.

The cutter used was a Taylor Stiles (Model No. TS-06) wet cutter with 6inch rotor blades operating at a rotor speed of 9,600 rpm. and equippedwith a 60 P screen having a 0.17 mm wide opening across the discharge ofthe cutter.

The polymer was mixed with water at 8C. and at that temperature waspassed through the cutter at a water flow rate of 1,360 liters per hourand polymer feed rate of 227 kilograms per hour. The resultant finelyground polymer had a particle size of 35 microns, an apparent bulkdensity of 417 grams per liter and a moldability index of 41.

The finely ground polymer was mixed with ten parts of water per part ofpolymer and agitated in a slurry tank for 6 hours at 10C. The slurrytank was equipped with vertical baffles to increase turbulence andjacketed to control the temperature. The tank had a diameter of 45.7cm., a height of 45 .7 cm., the agitator blades were rectangular andfour in number each measuring 22.9 cm. in diameter and 5.1 cm.in'height, with a 45 pitch. The agitator speed was 400 rpm. Theresultant slurried fine powder had a particle sizeof 33 microns, anapparent bulk density of 517 grams per liter, and a moldability index of42.

EXAMPLE 2 The wet finely ground powder obtained in Example 1 afterdraining off excess water was recycled to the cutter of Example 1 andre-cut under the same conditions as described in the example, exceptthat the 60P screen was replaced by an 80? screen having openings 0.13mm. wide. The resultant re-cut finely ground powder had a particle sizeof 29 microns, an apparent bulk density of 435 grams per liter, and amoldability index of 22 which was about one-half of the moldabilityindex obtained for the first cycle of comminution and slurrying.

EXAMPLE 3 The wet, re-cut, finely ground powder of Example 2 wasrecycled to the slurry tank of Example 1 and slurried under the sameconditions as described in Example 1. The resultant fine powder had aparticle size of 30 microns, an apparent bulk density of 501 grams perliter, and a moldability index of 18, illustrating retention of the goodmoldability index of the polymer and improvement in apparent bulkdensity.

EXAMPLE 4 In this Example, the granular polytetrafluoroethylene was thesame as the starting material used in Example 1. The polymer was finelyground in the type of mill described as a Hurricane Mill in col. 4 and 5of the Thomas and Wallace patent, using nitrogen instead of air andoperating at about 10C. to give finely ground powder having a particlesize of 36 microns, an apparent bulk density of 370 grams per liter anda moldability index of 7.

The powder was divided up into 200 gram samples for determining theeffect of slurrying temperature. Each sample was added to one liter ofwater and slurried in a 2-liter kettle equipped with 1.3 cm. widevertical baffles and a four blade agitator with 7.6 cm. diameter X 1.3cm. pitched blades rotating at 1,500 rpm. The temperature of the waterand agitation time and results are shown in the following table:

These results show even at prolonged slurrying at low temperature(10C.), neither the moldability index nor particle size is adverselyaffected as compared to slurrying for shorter times at highertemperatures (30C. and above). These results also show the generalincrease of moldability index with slurrying temperature at 35C. andabove, even at relatively short slurrying times, indicating anincreasing lack in quality of the molded article obtained from theslurried powder.

In each of the foregoing Examples, prior to measuring the parameters ofthe polymer, the polymer was dried in a circulating hot air oven at180C. for four hours.

As many apparently widely different embodiments of this invention may bemade without departing from the spirit and scope thereof, it is to beunderstood that this invention is not limited to the specificembodiments thereof except as defined in the appended claims.

1 claim:

1. A process for increasing the apparent bulk density of granularpolytetrafluoroethylene comprising comminuting thepolytetrafluoroethylene to have a particle size of less than 100 micronsand agitating the resultant polytetrafluoroethylene in an aqueous mediumat a temperature of less than 40C.

2. The process of claim 1 wherein the temperature is no greater than 3C.

3. The process of claim 1 wherein the particle size is from 10 tomicrons.

4. The process of claim 1 followed by at least one additional cycle ofcomminution and agitation as set forth in claim 1.

5. The process of claim 1 wherein the comminution is carried out at atemperature in the range of to 5 6. The process of claim 5 wherein thecomminution is carried out at a temperature of less than C.

7. The process of claim 1 wherein the agitation is sufficient toincrease the apparent bulk density of said tion thepolytetrafluoroethylene has a particle size of at least 500 microns.

* i v I I I!

2. The process of claim 1 wherein the temperature is no greater than3*C.
 3. The process of claim 1 wherein the particle size is from 10 to75 microns.
 4. The process of claim 1 followed by at least oneadditional cycle of comminution and agitation as set forth in claim 1.5. The process of claim 1 wherein the comminution is carried out at atemperature in the range of 0* to 40*C.
 6. The process of claim 5wherein the comminution is carried out at a temperature of less than15*C.
 7. The process of claim 1 wherein the agitation is sufficient toincrease the apparent bulk density of said polytetrafluoroethylene to bein excess of 450 g/l.
 8. The process of claim 1 wherein the agitation isconducted so that neither the moldability index nor particle size 23 soof the polytetrafluoroethylene increases by more than 20 percent.
 9. Theprocess of claim 1 wherein before comminution thepolytetrafluoroethylene has a particle size of at least 500 microns.